This invention relates to a thermoformable composite sheet having a major base layer of fiber-reinforced poly(ethylene terephthalate), sometimes hereafter referred to as PET, and a relatively thin layer of a thermoplastic adhesive material firmly attached to one side thereof. This invention also includes a process for making such thermoformable composite sheets.
Three-dimensional hollow articles can be thermoformed either in one operation, as taught, for example, in U.S. Pat. No. 3,953,358 to Wyeth or in separate operations, where parts are shaped as required, then bonded together into the desired article. A single mold process, which is preferred, still requires at least two separate thermoformable sheets, one of which may form a face member and the other one a backing member, which must be bonded to each other. Bonding may sometimes be accomplished by simply contacting both members in partly molten state under pressure. However, PET does not well adhere to itself at molding temperatures since molding is carried out well below its melting point. Self-bonding is even more difficult when PET is fiber-reinforced for greater rigidity. It is therefore necessary to use an adhesive material capable of bonding in a mold two fiber-reinforced PET sheets to each other. Such a material must adhere well to PET, be reasonably stable under the temperature and pressure conditions encountered in thermoforming, and bond thermoformed sheets into structures which will not deform or warp on cooling.
It also would be desirable to have available for thermoforming operations fiber-reinforced PET sheets having a layer of adhesive attached to one side thereof, so that coating individual sheets just prior to thermoforming would not be necessary.